1. Field of the Invention
The present invention relates to a semiconductor device.
2. Description of the Background Art
There are mainly a current-driven semiconductor such as a bipolar transistor and a voltage-driven semiconductor such as an insulated gate field-effect transistor (MOSFET) in a conventional semiconductor device.
While a greatest advantage of the bipolar transistor is to keep an ON-state resistance low by conductivity modulation, the bipolar transistor disadvantageously has difficult controllability and can not turn off at a high speed. On the other hand, while the MOSFET advantageously has excellent controllability and can turn off at a high speed, an ON-state resistance is disadvantageously increased when the MOSFET tries to have a high breakdown voltage capability.
An insulated gate bipolar transistor (IGBT) used since the 1980's is a bipolar device controlled by a MOS gate and has a structure in which a p-type collector layer is added to a side of an n-type drain layer of a vertical power MOSFET. Thus, controllability by voltage drive and a low ON-state resistance by double injection of carriers can be simultaneously achieved, however, high speed turning off is disadvantageously difficult.
In this regard, as a technique of simultaneously achieving the high speed turning off and the low ON-state resistance, there is a semiconductor device combined with the MOSFET and the bipolar transistor. This conventional semiconductor device comprises an npn bipolar transistor, an NMOSFET and a PMOSFET. A source and a drain of the NMOSFET are connected to a base and a collector of the bipolar transistor respectively. A source and a drain of the PMOSFET are connected to the base and an emitter of the bipolar transistor. The source of the NMOSFET is connected to the source of the PMOSFET. The gate of the NMOSFET and the gate of the PMOSFET are electrically connected to a gate terminal. In this semiconductor device, when the gate terminal is high, the NMOSFET is turned on, thereby turning on the bipolar transistor. Thus, an ON-state resistance is made lower. When the NMOSFET is in an OFF-state, the PMOSFET is turned on, whereby carriers stored at a time of an ON-state can be pulled out from the bipolar transistor. Thus, the high speed turning off can be achieved.
In the aforementioned semiconductor device combined with the NMOSFET, PMOSFET and the bipolar transistor, however, the sources of the NMOSFET and the PMOSFET are connected to the base of the bipolar transistor and hence the threshold voltages of the NMOSFET and the PMOSFET are disadvantageously unstabilized. Thus, an operation of the semiconductor device is disadvantageously unstabilized. Additionally, all terminals can not be formed on one surface because of a wiring structure, and hence integration with other devices is disadvantageously difficult.